We report an investigation of water and methanol transport in polymer electrolyte membranes based on highly sulfonated polyarelenethioethersulfones (SPTES) for direct methanol fuel cell (DMFC) applications. Measurements of both water and methanol self-diffusion coefficients of SPTES polymer as well as in a reference sample of Nafion- 117 equilibrated in 2 M methanol solution have been carried out, using the pulsed gradient spin echo technique, over a temperature range of 20-140 degrees C. The selectivity of the membrane, defined as (DOH/DCHA decreased from 6 to 2.4 as temperature increased from 20 to 140 degrees C in SPTES sample while in Nation, the value decreased from 3.2 to 1.4 as temperature increased from 20 to 100 degrees C. These results indicate significantly lower fuel molecular permeability in SPTES compared to that of Nation. All results suggest high-temperature stability in these materials, offering the possibility of fuel cell operation at temperatures > 120 degrees C. High pressure NMR diffusion measurements were also carried out for three different water contents (between 20 and 55 wt.%) in a static field gradient in order to get supplemental information regarding water transport in SPTES materials. The calculated activation volume increased from 1.54 to 8.40 cm(3)/mol as the water content decreased from 55 to 20%. This behavior is qualitatively similar to previously reported results for Nation-117. (c) 2005 Elsevier B.V. All rights reserved.